A continuing effort has been made over the years for developing methods and systems for removing pollutant gases from exhaust gases produced by combustion equipment. In recent years, environmental regulations have been made law in numerous countries around the world in an effort to reduce the emission of pollutant gases into the atmosphere from combustion equipment. Of major concern is the production of nitrogen oxides (NOx) by motor vehicles driven by internal combustion engines, such as gasoline driven engines, and particularly diesel engines. Other combustion apparatus are also of concern, such as furnaces installed in factories, commercial and home heating devices, power plant equipment, and so forth.
During the combustion process in such equipment when nitrogen in the air reacts with oxygen within a combustion chamber, under the high temperature and pressure conditions that typically exist therein, such as in the cylinder of an internal combustion engine, nitrogen oxides (NOx) are produced, which typically include either one or a combination of nitrogen monoxide and nitrogen dioxide, commonly referred to as NOx emissions. The NOx emissions are major atmospheric pollutants that cause smog, and acid rain. The major industrialized countries throughout the world have instituted regulations for reducing NOx emissions.
As a result, a major effort has been ongoing over an extended period of time for developing methods and systems to substantially eliminate the emission into the atmosphere of nitrogen oxides or NOx via exhaust gas streams from combustion equipment. Recognizing that automobile emissions are a major source of air pollution, in the 1966 automobile model year, the state of California passed regulations requiring the use of exhaust emission control systems in vehicles sold in California. Similar regulations were instituted throughout the United States by automotive model year 1968.
In combustion processes, the “perfect mixture” of a fuel and air is referred to in thermodynamics by the term “stoichiometric.” This is the point at which the amount of air is just enough to combust all of the fuel, with no excess oxygen remaining. For many reasons, internal combustion engines cannot be run stoichiometrically, and are typically run lean, where there is an excess of oxygen to fuel relative to the stoichiometric condition. Although both gasoline and diesel internal combustion engines are typically run as lean-burn engines, such a condition is most often found in operating diesel engines, and leads to the emission of undesirable amounts of NOx in the exhaust gases from such engines. At times engines may run rich, that is with an excess of fuel relative to oxygen. Note that for gasoline, the stoichiometric mixture is 14.6:1. Even under these conditions, some nitrogen from the air can react with oxygen to form NOx.
The exhaust gas stream from lean-burn engines contain significant amounts of oxygen, thereby preventing the efficient removal of NOx from the gas stream through use of conventional exhaust catalysts such as a “3-Way Catalyst.” As a result, NOx trap or NOx storage/reduction systems have been developed to assist in removing NOx from current lean-burn engines. However, these systems must rely on close engine control for alternating between rich and lean conditions in the exhaust gas stream. During the lean phases, the catalyst employed stores NOx. During the rich phases, the catalyst reduces NOx to N2. Also, HC-SCR systems have been developed as retrofits for use in reducing NOx from the exhaust gas stream of internal combustion engines, but such systems have found only limited use.
An attractive aftertreatment technology for active NOx control from an implementation point of view is one based on a catalyst system that utilizes on-board hydrocarbon (e.g. diesel fuel) as the source of the required supplemental reductant. The advantages of a Lean-NOx catalyst technology are its simplicity in terms of engine control, aftertreatment system and infrastructure implementation requirements compared to other technologies, such as NOx adsorbers or urea SCR. However, the state-of-the-art Lean-NOx technology for diesel engine applications is only able to achieve approximately 25-30% NOx reduction when supplemental diesel fuel is injected into the exhaust stream. Zeolites and noble metal based catalysts are the most common catalyst materials for Lean-NOx technology developed by catalyst suppliers for real applications with full size bricks. Poor hydrothermal stability, low NOx reduction selectivity and high sulfate formation are major issues of these materials for application as NOx control of diesel exhaust, particularly when applied to diesel exhaust at engine modes that produce low temperatures.
Accordingly, there is a need in the art for improved methods and apparatus for removing NOx from the exhaust gas stream of a combustion device, such as internal combustion engines.